Journal of Geotechnical and Geoenvironmental Engineering

This paper provides a new methodology for determining soil water content and density under different compaction energies. A new calibration equation was introduced making use of the voltage drop associated with the first passage of the electromagnetic wave through the soil specimen, the final voltage level after wave propagation ceases, and dry density normalized by the density of water. The new calibration equation allows for the direct calculation of dry density. Values of dry density are then used in the normalized apparent dielectric constant equation to determine the water content. Results of numerous laboratory tests on a wide variety of soils (including coarse-grained soils) showed that the proposed method accommodates different levels of compaction energy. The method also was validated with different field tests at sites commonly used in geotechnical earthwork construction, using soil-specific model coefficients determined from laboratory compaction tests. The results of this study indicate that the new calibration relationships appear relatively independent of the effects of compaction energy and of probe configuration in time domain reflectometry (TDR) testing. The proposed method presents an improvement to the one-step TDR method.

The waveforms from time domain reflectometry (TDR) tests on soil depend on the temperature of the soil being tested. This study provides a discussion of, and the basis for, temperature corrections for the parameters measured in the TDR test for use in calculating water content and dry density of soil. This paper is a companion to one that provides the basis for an updated ASTM Standard, but does not cover temperature effects. A brief synopsis of the new methodology is provided herein to identify parameters that are sensitive to temperature of the soil being tested.

Large cyclic loading on ballasted railroad tracks is now inevitable owing to an increased demand for freight and public transport. This leads to a progressive deterioration and densification of railroad ballast and consequently to the loss of track geometry and differential settlement. Understanding these complex stress-strain and degradation mechanisms is essential to predict the desirable track maintenance cycle, as well as the design of new track. This paper presents the results of cyclic drained tests and numerical studies carried out on a segment of model railway track supported on geosynthetically reinforced railroad ballast bed. The relative performance and effectiveness of single- and dual-layer configurations of geosynthetic reinforcement was evaluated using a large-scale prismoidal triaxial chamber. Laboratory tests on unreinforced and reinforced railway track were simulated in a numerical model, and the results were then analyzed to better understand the distribution of displacements and stresses inside the railroad ballast layer. It was observed that in view of strain and breakage control, both the type of reinforcement and its layout played a vital role in improving the capacity of the track. These laboratory test findings were supported by the predictions from an advanced elastoplastic numerical analysis.

This paper describes a theoretical model to capture the rate of erosion of a silty sand based on the principle of conservation of energy. Erosion is considered to begin when the interparticle bonds between grains are broken by hydrodynamic stresses exerted on the soil particles. These detached particles are then suspended and transported by the flow of eroding fluid. It is further assumed that once the particles are fully suspended and have reached the flow velocity, resettlement does not take place. Stabilization of soil particles because of lignosulfonate (LS) treatment is represented by the increased strain energy required to break the interparticle bonds. The equation proposed in this study is based on the shear stress-strain characteristics, mean flow velocity, mean particle diameter, and the packing arrangement of particles. The result of the proposed study is presented in the form of erosion rate versus the hydraulic shear stress. The model is validated with a series of laboratory erosion tests using the Process Simulation Apparatus for Internal Crack Erosion (PSAICE) for different percentages of LS. The model results are in good agreement with the experimental observations.

High spatial resolution ($\text{∼13.7 mm/pixel}$) neutron tomography was performed on partially water-saturated compacted silica sand specimens with two different grain morphologies (round and angular) at Helmholtz Zentrum Berlin using cold neutrons at the cold neutron radiography and tomography beam line. A specimen mixed with heavy water was imaged for contrast comparison purposes. Microfocus X-ray imaging was also performed on these specimens with slightly higher resolution ($\text{∼11.2 mm/pixel}$) using geometric magnification to locate the solid phase (silica particle boundaries) more precisely. Image processing was performed to remove unwanted gammas detected because of the gadox scintillator used for the high-resolution neutron imaging system. The visualization of solid, gas, and liquid phases for different grain morphologies is presented at the grain level. Using dual-modal contrast possible from simultaneous use of neutrons and X-rays, the authors introduce, for the first time, an improved ability to distinguish solid silica, liquid water, and gas phases. Quantitative analysis using three-dimensional tomography data is demonstrated for obtaining void ratio, void percentage variation over the height, and particle size distribution.

A model for the water retention behavior of unsaturated granular soil is developed by extending the classic bundled cylindrical capillary representation of pore space to a geometry more closely approximating that of granular porous media. Expressions for pore-scale saturation are derived as functions of matric suction for a three-dimensional unit pore comprising angular pore space bounded by spheres in simple cubic packing order. Water retention curves are modeled by assigning a statistical distribution of pore sizes optimized to best match experimentally determined retention curves. A key model attribute is its capability to capture evolution of fluid partitioning along drainage or wetting paths by differentiating pore water retained as thin films adsorbed to particle surfaces, liquid bridges retained in wedge-shaped pores, and saturated pockets in relatively small pores. Interfacial surface tension, solid-liquid contact angle, wetting direction, and mineralogy (Hamaker constant) are treated as model variables to examine their influences on water retention. Modeled retention curves compare well with measured curves obtained for glass beads, natural sands, and soils from the literature. The model effectively estimates hysteretic wetting-drying response, surfactant-induced surface tension lowering, and air-water interface area as a function of saturation. Capability to model evolution of fluid distribution has potentially important implications toward more robust understanding of macroscopic hydraulic, electrical, thermal, and mechanical behavior of unsaturated soil.

Dynamic compaction (DC) has an extensive performance record on municipal solid waste (MSW) sites. A database of 56 test sites has been developed and analyzed to garner lessons on the performance of DC on MSW sites. Analysis of the data indicates that the depth of improvement in MSW is shallower than the depth of improvement in cohesionless soils with values of the empirical parameter, $n$, for MSW commonly on the order of $0.4\mathrm{\pm }0.05$. DC results in a denser waste matrix that increases the stiffness of the waste by a factor of 2.8–24 compared with its unimproved state and results in reduction of waste compression upon load application. The achieved higher density and more stable structure of the MSW result in a lower rate and amount of long-term settlement. Limited studies indicate that DC is an effective technique to reduce settlements. Surface wave seismic methods represent an attractive method to assess the depth of improvement. Vibrations induced by DC are attenuated faster in MSW than in inorganic soils.

The effectiveness of layered-geotextile protection layers comprised of combinations of nonwoven needle-punched, woven slit-film, and nonwoven heat-bonded geotextiles to minimize strains in landfill geomembranes is examined. Results from physical experiments are reported where a sustained 700-N force was applied to a 28-mm-diameter machined steel probe on top of the protection layer, which was above a 60-mm-diameter, 1.5-mm-thick high-density polyethylene geomembrane and a 50-mm-thick compressible clay layer. The experiments are intended to simulate the physical conditions in a medium-sized landfill with an average vertical stress of 250 kPa and to capture the mean response with nominal 50-mm coarse gravel above the geomembrane. Screening tests were first conducted for up to 100 h at temperatures up to 55°C to evaluate three different combinations of layered geotextiles. Of those examined, the combination with a low-slack, heat-bonded geotextile above and below a thick, nonwoven, needle-punched geotextile as its central core was found to provide the lowest strains. A time-temperature superposition method was then developed and validated as a means to predict the long-term effectiveness of the most promising layered-geotextile composite. Last, long-term predictions of tensile strain were made and compared with proposed allowable limits. Despite the encouraging results from the short-term screening tests, even the most promising layered-geotextile composite is not recommended as a protection layer to limit long-term geomembrane strains for the particular force, particle size, and materials examined because the predicted strain after 100 years at 22–55°C of $\text{∼10\%}$ exceeds the range of currently proposed limits of 3–8%.

Liquefaction of sand is the main cause of failure for earth structures during earthquakes. Some past research showed that the fabric anisotropy of soils in the ground can cause different responses to both static and dynamic loading. The inherent fabric anisotropy of sand due to the deposition history and pattern may also affect the liquefaction potential of the ground. In this study, a group of earthquake centrifuge tests was performed on models of a level ground to investigate its seismic response with the soil deposited by different angles. A special rigid container was designed and used to prepare the models. It was found that the fabric anisotropy greatly influenced the response of the ground in displacement, acceleration, and excess pore pressure. The models with larger deposition angles had higher liquefaction potential. The results show that liquefaction evaluation of a sand layer should take into account the influence of fabric anisotropy caused by its deposition history and pattern.

The utilization of shear wave velocities toward the evaluation of the in situ geostatic horizontal stress state in soils is validated, specifically the lateral stress coefficient $K_0$. Field shear wave velocities from paired sets of different directional and polarization modes are compiled from 16 well-documented test sites involving a variety of geomaterials. Focus is particularly placed on shear wave velocities measured by downhole tests ($V_{s\text{VH}}$), crosshole tests ($V_{s\text{HV}}$), and special rotary-type crosshole tests ($V_{s\text{HH}}$). At these sites, field $K_0$ stress states have been quantified using one or more direct assessment techniques, including self-boring pressuremeter, total stress cells, and hydrofracture in field testing, as well as suction measurements, special consolidometers, and/or triaxial arrangements on undisturbed samples in the laboratory. Although the specific delineation of stress-induced versus inherent or fabric anisotropy may be difficult, it is shown that the ratio of horizontally polarized to vertically polarized shear waves (i.e., either $V_{s\text{HH}}\text{/}{V}_{s\text{VH}}$ or $V_{s\text{HH}}\text{/}{V}_{s\text{HV}}$) can be used to provide an approximate assessment of $K_0$ in soils, especially if coupled with the age of the formation.

The U.S. Army Corps of Engineers (USACE) conducted a load test along a portion of the London Avenue Canal as a part of an extensive study following the events of Hurricane Katrina to investigate the performance of I-walls and levees. A 45.7 m (150 ft) long section of the I-wall was hydraulically loaded, and pore pressures and displacements were measured in the vicinity of the I-wall through an extensive instrumentation system. The acquired data were assessed to develop a seepage model that would be applicable to the London Avenue Canal and areas where the soil conditions are similar. Two-dimensional and three-dimensional finite-element seepage analyses were conducted, a parametric study was performed, and a statistical calibration technique was developed to better estimate the actual field conditions in the numerical modeling. Based on this analysis, soil properties and finite-element boundary conditions are recommended for future analyses of the London Avenue Canal and other canals having similar subsurface conditions.

The one-way cyclic triaxial test has been recognized as a useful tool for solving many engineering problems, such as the prediction of deformation in subsoils under traffic loading. Most studies of the one-way cyclic behavior of soils are performed in triaxial tests with constant confining pressure (CCP), which can apply cyclic compressive deviatoric stress only. However, the real stress field in subsoils under traffic loading includes varying normal stresses in three directions. Therefore, it is more desirable to simulate the in situ loading conditions induced by passing vehicles through a combination of the varying deviatoric stress and varying confining pressure. Recognizing this, two series of one-way cyclic triaxial tests on remolded saturated clay were conducted in this study; one in undrained conditions and the other in partially drained conditions. In each series, both tests with the CCP and variable confining pressure (VCP) were carried out. Comparisons between the results of the VCP and CCP tests show that the VCP plays an important role in the one-way cyclic behavior of saturated clay, in terms of excess pore-water pressure, permanent volumetric strain, and permanent axial strain. Specifically, in partially drained conditions VCP tests deliver much larger permanent axial strain than CCP tests, indicating that the conventional CCP tests may underestimate the settlement of subsoils induced by traffic loading.

This paper examines the effect on the compressibility of a Portuguese soft soil stabilized with binders, with a special emphasis on creep characteristics. The effects of vertical stress during curing time and binder composition (type and content) on the creep behavior of a stabilized material are analyzed with oedometer tests. The results show the decrease of the secondary compression index with the decrease of the vertical stress applied during creep phase and with the increment of the binder content. The best binder type for this soil, in terms of creep behavior, is composed of portland cement and fly ash with a dry weight proportion of 75/25, because this promotes the development of secondary pozzolanic reactions. The results also reveal that the binder composition should have a minimum quantity of cement to improve the compression characteristics of the stabilized soil adequately.

Underground tunneling, besides raising geotechnical challenges, can often modify the hydrological regime around an excavated area. This fact entails not only the modification of superficial aquifers but also the appearance of subsidence phenomena as a consequence of the decrease in pore pressure. In the end, this may jeopardize the structural integrity of the construction, leading to delays and undesirable secondary effects. This paper documents the problems arising from the construction of a railway tunnel for a high-speed train in the province of Orense in the northwest of Spain. The tunnel was excavated through a highly weathered granitic massif using conventional methods. The drawdown in the water table caused by the tunnel advance was analyzed and monitored. The initial lowering of the water level was followed by a period of soil subsidence with deflections reaching up to 104 mm. These differential settlements happened along a 150-m strip on both sides of the tunnel axes, which affected urbanized areas. The topographic downfall was located ahead of the perforation head; however, it occurred after the lowering of the water table as an immediate consequence.

Soils in the field or at reclaimed sites commonly contain some amount of silt or clay rather than clean sand only. The effect of a low fraction of plastic fines within sand matrix on liquefaction resistance is not clearly understood. In this study, clean sand was mixed with 10% plastic fines having different plasticity indexes (PIs), and the effect on liquefaction resistance was evaluated in terms of cyclic stress ratio. A series of undrained cyclic triaxial tests were carried out on loose, medium, and dense specimens that were reconstituted in the laboratory by the undercompaction method. The results showed that liquefaction resistance tended to decrease as the PI of 10% fines in the specimens increased. Liquefaction resistance of loose specimens was marginally influenced by the plasticity of fines. However, in the case of dense specimens, liquefaction resistance decreased up to 40% as the PI of 10% fines increased. It was shown that even though a low fraction of plastic fines was included within sand matrix, it still had a significant effect on the liquefaction resistance of sandy soils.

Kovacs’ criterion to assess the potential for internal instability of cohesionless soils that was proposed, but never tested experimentally, is verified with reference to a database of laboratory test results. The inherent margin of safety is described, and a threshold boundary between stable and potentially unstable gradations is established. A variation on the method is then presented, termed the simplified-Kovacs criterion.